Inductors are used in many of today's signal processing circuits. For example, in diplexer and triplexer circuits, inductors and capacitors are used to implement the signal filters that are part of the diplexers and triplexers.
Typically, when an inductor is implemented in a circuit, a discreet inductor component is used. Generally, there are two types of discrete inductors. A first type is a fixed-value sealed inductor in which the inductance value of the inductor is fixed (i.e. not adjustable). This type of inductor is commonly implemented as a coil or winding of wire around a core, which may be made of various types of material. Due to manufacturing variations, material variations, etc., the best achievable tolerance for this type of discrete inductor is approximately 2%. This means that the inductance value of an inductor of this type can be precise to within 2% of a target inductance value (thus, the actual inductance of the inductor may be exactly the target inductance value or it may be up to 2% off of the target inductance value). This relative lack of precision may render the fixed-value inductor unusable in some applications. The other type of discrete inductor is a variable inductor, which has windings that are slightly spread open so that they can be adjusted. By spreading the windings, the inductance can be decreased. Conversely, by compressing the windings, the inductance can be increased. Because this type of inductor can be adjusted, a very precise inductance value can be achieved. However, because the windings require manual adjustment, the process of achieving the desired inductance value can be quite labor intensive.
Diplexer and triplexer circuits usually require sharp-cutoff signal filters that can change their amplitude response very quickly as frequency changes. For this type of signal filter, inductors with very precise inductance values are needed. Because of their lack of precision, fixed-value discrete inductors are typically not suitable for diplexers and triplexers. As a result, most diplexers and triplexers are implemented with variable inductors. As noted above, however, discrete variable inductors require manual adjustment, which can be quite labor intensive. This labor slows down the manufacturing process and significantly increases the cost of the final product.
Another drawback to the use of discreet inductors (either the fixed-value type or the variable type) is that they tend to require significant amounts of space. With devices becoming ever smaller, space is a precious commodity that needs to be conserved whenever possible. Thus, anything that requires large amounts of space is generally disfavored.
Given the drawbacks of using discreet inductors, an improved technique for implementing an inductor in a circuit is needed.